The present invention relates to steam turbines and, more particularly, to a valve system for controlling steam turbines.
The output torque of steam turbines is conventionally controlled by a set of control valves which are opened in a fixed sequence by a sequential cam-operated mechanical controller. Such a sequential cam-operated controller is disclosed in U.S. Pat. No. 4,082,115, the disclosure of which is herein incorporated by reference.
A partially opened control valve for a steam turbine produces a pressure drop thereacross which adversely affects the efficiency of the steam turbine. Such a control valve is at its most efficient when it is fully open. In addition, the smaller the flow capacity of a fully open control valve, the less its inefficiency. The above referenced cam-operated control valve partially solves the efficiency problem by separating the required valve capacity into a plurality of substantially equally sized smaller valves. The valves are opened in sequence, with one valve at a time being varied to govern the flow and all of the others either fully opened or fully closed (except for a slight transition overlap during which one valve is not quite in the fully opened condition but is no longer considered to be exercising effective control over the steam flow). Thus, at most, the inefficiency experienced is only that due to a single one of the plurality of control valves.
A valve characteristic of conventional control valves produces a non-linear relationship between valve position and steam flow. The mechanically driven cams of the above-referenced sequential control valve system are shaped to compensate for the valve characteristic whereby a given increment of rotation of the set of cams produces a linear increment of change in steam flow.
Although the sequential cam-driven control valve system of the prior art exhibits improved partial-load efficiency over a single large control valve, the rigid sequential operation of such a control valve system requires that all of the control valves have substantially equal flow capacities.
U.S. Pat. No. 4,325,670 discloses a valving system for a steam turbine which employes unequal valve sizes actuated in a sequence which improves efficiency. The operating sequence in the referenced patent permits simultaneous governing by two valves and fails to disclose a method for always using the smallest-capacity one of the available control valves.